EP0353153A1 - Magnetic oscillation and guiding device for charged particles for the amplification of an electromagnetic emission - Google Patents

Abstract

Magnetic oscillation and guiding device for charged particles for the amplification of an electromagnetic emission. The device comprises a succession of electrically conducting flat coils (Ei, Ei+1,...) along the axis (Z) corresponding to the mean trajectory of the particles. Each coil surrounds this axis. The respective planes of these coils are alternately inclined to one side and the other of the said axis so that the assembly of coils generates, at one and the same time, magnetic fields for oscillation (BT) and for guiding (BP) of the particles when these coils are supplied with electric current in such a way that they exhibit the same magnetic polarity on a same side of the said axis. Application to the amplification of microwaves and to the free- electron laser. <IMAGE>

Description

The present invention relates to a magnetic oscillation and guidance device for charged particles, intended for the amplification of electromagnetic radiation. It applies in particular to the production of a microwave amplification system or of a free electron laser.

Information on these systems is given in particular in the article by J.M. ORTEGA and M. BILLARDON which may be referred to and which is entitled "The free electron laser" and published in the journal Pour la Science, April 1986, pp. 91 to 100.

We know that in such systems, a periodically variable magnetic field B _T , applied transversely to the axis Z of propagation of a beam of relativistic electrons, makes it possible to transfer the energy of the latter to coherent electromagnetic radiation Re under certain conditions. The electrons are forced to perform oscillations in an XZ plane perpendicular to the magnetic field B _T , the latter remaining parallel to an axis Y which is perpendicular to the XZ plane as seen in Figure 1.

It is known to generate these oscillations by means of permanent magnets 2 which are arranged periodically along the axis Z and whose north N and south S poles are alternated (see Figure 1). The variable magnetic field B _{T is} thus obtained along the Z axis and parallel to the Y axis which is perpendicular to Z, the latter being perpendicular to X.

We also know that the same result can be obtained by means of electromagnetic coils (not shown) arranged alternately in opposite directions along the Z axis (average trajectory of the electrons), the axes of these coils being all parallel to the axis Y.

The desired oscillations are thus obtained. However, the electron beam does not propagate properly under these conditions due to phenomena of space charges and various instabilities. It is necessary to add to the corrugation device (constituted by the permanent magnets or by the electromagnetic coils) a guiding device which is generally constituted by a set of magnetic quadrupoles or by a solenoid 4 which produces a magnetic field B _P parallel to the electron propagation Z axis.

Known oscillation and guidance systems are therefore complicated and expensive since they require two separate devices, one for oscillation and the other for guidance.

The object of the present invention is to remedy this drawback by proposing a single device capable of producing both the magnetic oscillation field B _T and the magnetic guide field B _P.

Specifically, the subject of the present invention is a device for magnetic oscillation and guidance, along an axis, of charged particles, intended for the amplification of electromagnetic radiation, this device being characterized in that it comprises a succession of electrically conductive flat windings along said axis, each winding surrounding the latter, the respective planes of the windings being alternately inclined by a side and the other of said axis, so that all of the windings generates both magnetic fields of oscillation and of guiding the particles when these windings are supplied with electric current in such a way that they have the same polarity magnetic on the same side of said axis.

The device which is the subject of the invention may have a constant pitch or else may advantageously have a continuously variable pitch, the pitch being the distance which separates two windings from the device, which are of odd rank and which are separated by a single winding (of even rank), the first winding of the device being for example that which is first encountered by the particles when they enter the device.

In the device which is the subject of the invention, each winding can form a short solenoid or a spiral.

In addition, the windings can be mounted in series or in parallel or the set of windings can be organized in groups of adjacent windings, the windings of each group being mounted in series and the groups being mounted in parallel.

In addition, in the device which is the subject of the invention, each of the windings of even rank which has two closest neighbors can be equidistant therefrom.

Finally, as already mentioned, the charged particles can be electrons.

• - la figure 1 est une vue schématique d'un dispositif connu d'oscillation et de guidage de particules chargées, destiné à l'amplification d'un rayonnement électromagnétique et a déjà été décrite,
• - les figures 2 et 3 illustrent schématiquement et partiellement un dispositif conforme à l'invention,
• - les figures 4 et 5 illustrent schématiquement des enroulements utilisables dans l'invention,
• - les figures 6 et 7 représentent respectivement les variations, en fonction de Z, des champs B_T et B_P que l'on obtient avec des enroulements de type "solénoïde court" agencés conformément à l'invention,
• - la figure 8 représente schématiquement un dispositif conforme à l'invention, à pas continûment variable, et
• - les figures 9 à 11 représentent des dispositifs conformes à l'invention, comportant des enroulements montés en série (figure 9), en parallèle (figure 10), ou organisés en groupes d'enroulements, ces enroulements étant en série et les groupes en parallèle (figure 11).

The present invention will be better understood on reading the following description of exemplary embodiments given purely by way of indication and in no way limiting, with reference to the appended drawings in which:

• - Figure 1 is a schematic view of a known device for oscillating and guiding charged particles, intended for the amplification of electromagnetic radiation and has already been described,
• FIGS. 2 and 3 schematically and partially illustrate a device according to the invention,
• FIGS. 4 and 5 schematically illustrate windings which can be used in the invention,
• FIGS. 6 and 7 respectively represent the variations, as a function of Z, of the fields B _T and B _P which are obtained with windings of the "short solenoid" type arranged in accordance with the invention,
• FIG. 8 schematically represents a device according to the invention, with continuously variable pitch, and
• - Figures 9 to 11 show devices according to the invention, comprising windings mounted in series (Figure 9), in parallel (Figure 10), or organized in groups of windings, these windings being in series and groups in parallel (Figure 11).

In Figure 2, there is shown schematically and partially a device according to the invention. This device comprises a plurality of electrically conductive planar windings which extend along the axis Z (average trajectory of the electrons). Each winding surrounds this axis Z. Only a few windings Ei-2, Ei-1, Ei, Ei + 1 and Ei + 2 are shown in Figure 2. Each winding is symbolized by a circular turn closed on itself, which is of course not the case in reality since the windings must be supplied with electric current (by means not shown).

When the device is operating, each winding is traversed by an electric current I, the direction of which is the same for all the windings: if the device is observed in the direction of increasing Z for example (direction of the progression of the particles in the device ), all the windings have the same magnetic polarity to the observer who thus sees either the north faces of the windings, or the south faces of these.

The center of each winding, denoted Oi for Ei, is on the Z axis and we define, for each winding, a normal axis denoted Ni for Ei, which passes through the center of this winding, is perpendicular to the plane of this last and oriented in the direction of the magnetic field which would exist at the center of the winding if the latter was alone. The axes thus defined Ni, Ni + 1, Ni + 2, ... are in the YZ plane.

We also define, for each winding, another axis denoted Ui for Ei, which passes through the center of the winding, is in the plane YZ and perpendicular to the corresponding normal axis.

The windings are arranged so that their planes are alternately inclined on one side and the other of the Z axis with a constant inclination: each of these planes or axes Ui, Ui + 1, Ui + 2, .. makes the same acute angle t with the axis Z and the respective normal axes of the windings are alternately in one of the half-spaces delimited by the plane XZ and in the other of these half-spaces.

In the example shown in Figure 2, the device has a constant pitch P. Thus, two windings inclined on the same side of Z and separated by a single winding, are separated by a distance P and the winding which separates them is equidistant from these two windings: by considering for example Ei-1, Ei and Ei + 1, the distance from Oi-1 to Oi + 1 is equal to P and the distance from Oi-1 to Oi is equal to the distance from Oi to Oi + 1 which is equal to P / 2.

In FIG. 3, the magnetic field B has been constructed resulting, at a point of Z, from two adjacent windings such as Ei and Ei + 1. The field Bi generated by Ei has a component B _Ni parallel to Ni and a component B _Ui parallel to Ui. Similarly, the field Bi + 1 generated by Ei + 1 has a component B _{Ni + 1} parallel to Ni + 1 and a component B _{Ui + 1} parallel to Ui + 1. Field B results from the composition of Bi and Bi + 1 and breaks down into B _T (parallel to Y) and B _P (parallel to Z).

We therefore see that, in accordance with the invention, with a single device, we obtain both the oscillation field B _T , which is parallel to Y and whose intensity varies sinusoidally in the YZ plane, and the guide field B _P which is parallel to Z and retains the same direction at any point of Z.

Each of the windings can be a very short solenoid, and therefore almost flat, comprising only a few turns, less than 10 turns for example (see FIG. 4 in which the means for supplying the windings with electric current are not shown).

As a variant, each winding can be in a spiral with a few turns of electrical conductor, less than 10 turns for example (see FIG. 5 in which the means for supplying the windings with electric current are not shown).

Furthermore, each flat winding, observed perpendicular to its plane, can be made up of circular or square or rectangular turns for example.

In an exemplary embodiment (given for purely indicative and in no way limiting) I = 20A, t = 60 °, each winding is a solenoid of 4 turns (of the kind of those of FIG. 4), with an internal diameter of 10 cm, and only 6 solenoids are used. The variations of the corresponding B _T field, as a function of Z, are shown in FIG. 6 and the variations of the corresponding B _P field, as a function of Z, are shown in FIG. 7.

The use of high pulse currents, the intensity of which can range from several kA to several tens of kA and whose pulses can have a period of the order of 1 ms to a few ms for example, makes it possible to obtain magnetic fields. intense, of the order of 1T or more.

The technique used in the present invention allows the construction of coils of large internal diameter relative to the diameter of the beam of charged particles. This technique also allows the use of slight inclinations (sharp acute angles t), which are compatible with a small distance between two adjacent windings in the device object of the invention.

In addition, the pitch P of this device is not necessarily constant, it can advantageously be continuously variable along the axis Z (see FIG. 8 on which the means for supplying the windings with electric current are not shown). Indeed, charged particles such as electrons, which pass through the device according to the invention (inverter) lose energy there as they progress along the Z axis. The phase agreement between particles are no longer respected. A continuously variable step allows to compensate for this, that is to say to maintain the phase agreement. To this end, the step is decreased in the direction of increasing Z (direction of progression of particles in the device). Indeed, the wavelength l of the radiation that we want to amplify is substantially equal to:
(1/2) .P. (1- (v / c) ²). (1+ (K² / 2)) with K = 93.4 B _T .P
P being expressed in meters, B _T in teslas, v (speed of electrons) in m / s and c (speed of light in a vacuum) in m / s.

Consequently, if the energy of the electrons decreases (from where a decrease in their speed v), it is necessary to decrease P so that l remains constant.

This is what we see in FIG. 8: the device schematically represented in this figure comprises n windings E1, ..., En (n being assumed to be odd for example) and the steps P1.3 (distance from E1 to E3), P3.5 (distance from E3 to E5) ... Pn-2, n (distance from En-2 to En) form a decreasing sequence in the direction of the Z axis.

Finally, the windings can be mounted in series (Figure 9) in order to supply the assembly thus obtained by a pulse current generator 6. It will be noted that we can compare this assembly to a solenoid whose turns are distributed in groups of adjacent and contiguous turns, these groups being non-contiguous and tilted alternately on one side and the other of the Z axis.

As a variant, the windings are mounted in parallel (FIG. 10) and supplied by the generator 6.

In another variant, groups 8 of adjacent windings are formed, in each group the windings are mounted in series and the various groups are mounted in parallel and supplied by the generator 6 (FIG. 11).

The device which is the subject of the invention therefore lends itself well to adaptations of the magnetic field B _T and of the distance between two adjacent windings along of the propagation axis Z of the particles.

The realistic configurations that can be envisaged lead to a distance d between two adjacent windings which can range from a few centimeters to around twenty centimeters, for example. The inside diameter di of the turns, for its part, is liable to vary within the same range of values. The minimum values of this distance d and of this diameter di are dependent on the transverse dimension of the particle beam. The maxima of the distance d and of the diameter di are dependent on the intensities of the magnetic fields desired and on the intensity of the supply current achievable.

The preceding orders of magnitude show that the device of the invention is applicable to the amplification, by means of charged particles such as electrons, of electromagnetic waves whose wavelengths can range from a few centimeters (hence a frequency of the order of a few GHz) to a few micrometers, depending on the energy of the electrons of course. The applications of the invention therefore relate to a very wide range of systems covering microwave amplification systems and free electron lasers.

Returning to the case of the use of a pulse current generator, it will be noted that the duration of a current pulse generated by this generator must be greater than the duration of the puff of particles, or of the train of puffs of particles. , which passes through the inverter. It is indeed necessary that the fields B _T and B _P are constant in time during the duration of this puff or this train of puffs (we know that each puff sent to the inverter is structured there in bundles of particles, by interaction with electromagnetic radiation). AT purely indicative and in no way limitative, if the duration of the puff of particles, or of the train of puffs of particles, is of the order of 1 microsecond, it suffices to keep the fields B _T and B _P constant for approximately 1 ms.

However, in the particular case of superconductive windings, it is possible to use a direct current generator in order to produce the fields Bet B _T. It will be noted that these must be at least equal to 0.1 T in the case of a free electron laser (using bursts of electrons or a continuous beam of electrons).

Claims (9)

1. Device for magnetic oscillation and guidance, along an axis (Z), of charged particles, intended for the amplification of electromagnetic radiation (Re), this device being characterized in that it comprises a succession of electrically conductive flat windings (E1, ..., En) along said axis, each winding surrounding the latter, the respective planes of the windings being alternately inclined on one side and the other of said axis, so that all the windings generate both magnetic fields of oscillation (B _T ) and guide (B _P ) of the particles when these windings are supplied with electric current in such a way that they have the same magnetic polarity as one same side of said axis. 2. Device according to claim 1, characterized in that this device has a continuously variable pitch. 3. Device according to any one of claims 1 and 2, characterized in that each winding forms a short solenoid (sl). 4. Device according to any one of claims 1 and 2, characterized in that each winding forms a spiral (sp). 5. Device according to any one of claims 1 to 4, characterized in that the windings (E1, ..., En) are mounted in series. 6. Device according to any one of claims 1 to 4, characterized in that the windings (E1, ..., En) are mounted in parallel. 7. Device according to any one of claims 1 to 4, characterized in that all of the windings are organized in groups (8) adjacent windings, the windings of each group being mounted in series and the groups being mounted in parallel. 8. Device according to any one of claims 1 to 7, characterized in that each of the even rank windings which has two closest neighbors is equidistant therefrom. 9. Device according to any one of claims 1 to 8, characterized in that the particles are electrons.

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